1. Field of the Invention
The present invention relates to a method for fabricating a laser diode, and more particularly to a method for fabricating a laser diode having planar buried heterostructure capable of modulating at high speed.
2. Description of the Prior Art
Generally, a laser diode, which is capable of modulating at high speed and thereby has been widely used as an optical source for an optical fiber communication, has a buried heterostructure. Such a laser diode having a buried heterostructure has some advantages such as low threshold current, high quantum efficiency, high characteristic temperature and so on. This is because in the laser diode a current blocking layer can be formed on both sides of an active layer formed between two clad layers so as to prevent current-leakage during operation.
FIGS. 1A to 1D shows the process steps for fabricating a conventional planar buried heterostructure laser diode.
Referring to FIG. 1A, on an n-InP substrate 1, a first clad layer 2 of n-InP, an active layer 3 having a multiple quantum well (hereinafter, referred to as "MQW") structure and a second clad layer 4 of p-InP are sequentially formed to complete a first crystal growth. The MQW structure is formed of undoped InGaAs/InGaAsP pairs. In the first crystal growth, the active layer 3 has the MQW structure formed by a metal organic chemical vapor deposition (hereinafter, referred to as "MOCVD").
As shown in FIG. 1B, on the second clad layer 4, a patterned mask layer 5 of SiO.sub.2 or Si.sub.3 N.sub.x (x.congruent.4) is formed by a well-known photolithography in the process. Using the mask layer 5 as an etching mask, the second clad layer 4, the active layer 3 and the first clad layer 2 are mesa-etched by an etching solution until the substrate 1 is exposed. In the etching process, a non-selectively etching solution has to be used because InGaAs and InGaAsP constituting the MQW structure of the active layer B are different etching speed in a selective etching solution. Also, the layers 2, 3 and 4 are isotropically removed by a wet etching method, and therefore a round-shaped structure, or a circumferential structure is formed in view of a cross-sectional profile.
In FIG. 1C, around the round-shaped structure, first and second current blocking layers 6 and 7 are formed to complete a second crystal growth. The first current blocking layer 6 is formed of p-InP and the second current blocking layer 7 is formed of n-InP. Also, the current blocking layers are formed by a liquid phase epitaxy (hereinafter, referred to as "LPE") method. Since the current blocking layers 6 and 7 are not grown on the mask layer 5 an upper surface of the second current blocking layer 7 is flat.
With reference to FIG. 1D, after removal of the mask layer 5, a third crystal growth is performed. Then, on the second current blocking layer 7, a third clad layer 8 of p-InP and an ohmic contact layer 9 of p-InGaAs are sequentially formed by the LPE method. Next, an n type electrode 10 is formed on the rear surface of the semiconductor substrate 1 and a p type electrode 11 is formed on a surface of the ohmic contact layer 9.
In the planar buried heterostructure laser diode which is fabricated in accordance with the above described method, a thyristor structure is provided which is constituted by the third clad layer, the second current blocking layer, the first current blocking layer and the substrate. Also, in the above mentioned thyristor structure, the first current blocking layer has to be doped higher in impurity concentration than the p type clad layer and formed relatively thick. The first current blocking layer having a high impurity concentration and a thick thickness, however, is lowered in resistance, and thereby a current can flow sufficiently through the first current blocking layer. Then, a leakage current flows in the interface between the substrate and the first current blocking layer. The leakage current is in proportion to a distance d between the active layer and the second current blocking layer, as shown in FIG. 1D. In order to reduce a leakage current flowing in the interface between the substrate and the first current blocking layer, the distance d has to be determined as short as possible.
However, in order to reduce a leakage current in the thyristor structure, if the first current blocking layer is formed relatively thick, the distance d between the active layer and the second current blocking layer becomes relatively long. This causes a serious problem that the leakage current is increased in the interface between the substrate and the first current blocking layer.
In addition, if the first current intercepting layer is formed relatively thin so as to reduce a leakage current in the interface between the substrate and the first current blocking layer, the distance d between the active layer and second current blocking layer becomes relative short. This causes another serious problem that a leakage current is increased through the thyristor structure.